1. Field of the Disclosure
Embodiments disclosed herein relate generally to downhole tools. More specifically, embodiments disclosed herein relate to hydraulic anchors and methods related to setting hydraulic anchors in a wellbore.
2. Background Art
The drilling industry often has the need to monitor the axial position and angular orientation of a tool (such as a whipstock) within a wellbore, and to rigidly secure the tool within the wellbore once a required position and orientation has been achieved. The position and orientation of a tool may be determined by using measurement-while-drilling tools (“MWD”), which require a flow of wellbore fluid through a drillstring to communicate a measured position and orientation to the surface. The flow rates required for adequate MWD communication may often be sufficiently high to generate a pressure drop between the inside and the outside of the drillstring, which may prematurely set a hydraulic anchor in the wellbore.
To overcome this problem, a drillstring may often include a bypass valve located between the MWD tool and an anchor (located downhole of the MWD tool). When the position and orientation of the drillstring is being monitored, wellbore fluid is pumped through the MWD tool via the bore in the drillstring. The bypass valve prevents the setting of the anchor by allowing the wellbore fluid flowing downhole of the MWD tool to pass out bypass ports and into the wellbore annulus before reaching the anchor. The fluid pressure differential across the hydraulic anchor is thereby maintained below the setting pressure.
Once the drillstring position and orientation is set, the hydraulic anchor may be set by increasing the flow rate of the wellbore fluid down the drillstring. The increase in flow rate causes an increase in dynamic pressure in the bore of the drillstring. When the dynamic pressure increases to a predetermined magnitude, the bypass valve is closed, which eliminates the fluid path between the wellbore annulus and the drillstring bore. Thus, the wellbore fluid is directed downhole to the anchor where the appropriate setting pressure (typically 1500-3000 psi differential between inside and outside of the anchor) is then applied to set the anchor.
A conventional bypass valve incorporates a piston which slides within a cylinder in response to dynamic wellbore fluid pressure. The wall of the cylinder is provided with a plurality of holes (i.e., bypass ports) through which fluid may pass from the drillstring bore to the wellbore annulus. The piston is held in an open position by biasing means, such as a spring or a shear pin, or a combination of both. When the appropriate dynamic pressure is achieved, the biasing means is overcome and the piston slides within the cylinder so as to sealingly close the bypass ports.
This type of bypass valve may be problematic when the wellbore fluid within the drillstring carries a large amount of debris. Such debris may be pumped from the surface, produced by a component failure in the MWD tool, or generated during the drilling of the wellbore. The debris may accumulate on the piston and increase the force exerted on the piston by any given flow rate of wellbore fluid. In certain circumstances, the accumulation of debris may be sufficient to cause the bypass valve to close prematurely. This in turn may cause a premature setting of the hydraulic anchor.
Multi-cycle bypass valves also use a biased piston to remain open. A multi-cycle bypass valve uses a guide pin to control the position of the piston. The piston has a slot for the guide pin travel. The piston will move towards the closed position each time flow is increased and the fluid pressure is sufficient enough to push it downward. The piston will move from an open position to a partially open position in response to pressure. However, the guide pin keeps the piston in a position such that the valve will not close until it has been cycled a predetermined number of times. The position and orientation of the drillstring may continue to be adjusted even though the bypass valve piston has stroked. However, multi-cycle bypass valves are limited themselves, in that the number of cycles is limited, and the guide pins may be unreliable.
Accordingly, there exists a need for a downhole tool that is capable of multiple cycles and that is not susceptible to being prematurely set in the wellbore by a fluid pressure increase.